Dell inspiron 1705 9300 17inch shuts down after 15-20 mins win xp

It sounds like your system might be running hot. Do you hear the fan running? If so it will turn on after it has cooled off, but once it gets hot it will turn off. If you immediately try to turn it back on, it won't work until the system has cooled off.

Laptops overheat because of the following problems:
1. Dust has clogged the Fans, Vents, CPU & Heat Sink Area
2. The FAN has stopped working
3. The Thermal Paste around the CPU and HeatSink has either dried up or is too thin to keep the system cool.

Try a can of compressed air and blow out the areas you can get to around the vents. You'll have to take the laptop apart to get to the Fan, CPU and Heatsink

I think you're right. Sounds as if the system is overheating. Check the fan is working. If it is, you will probably need to open the case and clean out all the vents & the fan. Remove the heatsink, clean off old thermal paste and renew before replacing the heatsink

Clean the CPU heatsink, it may become clogged with dirt and fluff. Open the door on the bottom of the PC and blow some air from the air vent in the direction of the cooling fan.

This sounds like an over heating problem, your laptop shuts down as a safety measure. The only solution for this is to have a professional service the unit.

you need to check a few things... when the fan/heatsink assembly were replaced, did you make sure that the thermal pads on the CPU and GPU are NEW/Good.... Make sure that the Heatsink GPU pad Firmly sits in the chip....

You can test the thermal controller by using a hair dryer or heat gun.... Point it onto the heatsink pipes and see if your fan spins when the pipes get hot, then remove and see if the fans slow after a minute...

You might have faulty RAM or Misseated RAM... Pull The RAM chips and reseat them...
Also Make Sure Your CPU Is Locked Down....

Periodic Shut Downs Can Be Caused By Quite A Few Different Things...

Typically it's due to Thermal Damage
Though Failing RAM or A Failing Hard Drive, Overheated Video/GPU Chip, Or Even Failing CPU can as Well Cause This Issue...

Please try to upgrade the BIOS version and flash the BIOS. Please check the link and follow the instructions: http://support.dell.com/support/edocs/systems/ins1525/en/SM/bios.htm#wp1179839

I think your right.Heating most of the time will cause a premature shut down.It may be your ram getting hot or video card and possibly your HDD.The ac adapter could also be overheating/going bad as well.You could blow it out through the vents, with it off and unplugged.You maybe able to get a notebook cooler to use with it.It MIGHT help.If you could get it apart and blow it out ,you may increase you chances.

Usually when a CPU overheats it shuts down and after it cools down you can re-boot it again.
Continual overheating will eventually cause the CPU to fail.

The overheating is usually caused by blocked vents and or dust build up on the heatsink and/or fan. Remove this dust from the heatsink and fan and this should fix your overheating problem, unless you have a faulty fan, in this case you will need to replace the fan.

Dell Inspiron 5150, has overheating problems. The position of the fan is a problem.Also,don't press of the left upper corner of the laptop.Here's an excerpt from a wikipedia article: "The overheating problems on the Inspiron 5150 model was due to the position of the fan and fan-vent and the way in which it vents air through the CPU's heatsink.[citation needed] The fan draws air from underneath the unit and vents it through the rear after passing over the heatsink. However, the unit also sucks up dust from underneath it. Over time a buildup of dust constricts the airflow through the unit, reducing cooling. The airflow is perceptibly diminished. A simple test is to place a hand near the fan vent; when the fan is running at high speed there should be a good flow of warm air. Heat will also build up along the top of the unit near the LCD display and left toward the optical drive. Users may notice this while typing. Performance may also suffer, with jittery game movement and slow Windows performance. Some systems may experience jerky reactions where the unit goes into an almost slow-motion phase for short periods of time. Continued operation in this condition may lead to overheating and could result in permanent damage to the CPU, GPU, motherboard and other internal components. To remedy this the unit must be disassembled, the dust removed, and fan operation checked. This problem not uncommonly results in a CPU temperature-increase of between 15-20°C over an normally functioning unit. Further temperature increases may damage the casing of the unit."From Wikipedia article"Dell Inspiron" under problems Hope it helps -CG

All fans for cooling cpu plug into a small 2 or 3 pin socket usually white on the motherboard near to the heatsink your fan should have the lead coming from it
fans for video card sometimes takes the power from card or connect to board same as cpu fan but near video card